A decade later as President,Jefferson worried whether the Constitution provided him the power to annex new territory--specifically, the Territory of Louisiana, which France offered to the United States for purchase.

Writing to a friend at the time, Jefferson expressed doubts about whether the Constitution enabled him to acquire the extensive new lands stretching across the vast middle of the continent.

In the end, however, the Louisiana Purchase was too great an opportunity to pass up. Jefferson's view of federal power became somewhat more expansive: he concluded that the Constitution implicitly allowed the United States to acquire territory.

Further Reading:

The Letter That Bought An Empire Written in haste, on an April midnight in 1803, the unedited text of the message that led to the Louisiana Purchase is printed for the first time.